DC offset/Blocking Caps for my preamp - diyAudio
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Old 12th May 2009, 04:51 PM   #1
dano12 is offline dano12  United States
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Default DC offset/Blocking Caps for my preamp

Can I assume that my butt simple one opamp preamplifier can do without an output cap?

It will be plugged into various consumer grade receivers and amplifiers I have.

What I don't know is whether or not said consumer grade devices would be sensitive to DC offset at their line level inputs (i.e. is the 'standard' to have input caps on all line input channels in consumer-grade gear?)...

The reason I'm asking is to try to reduce the number of capacitors in the signal path, and if I can do away with the output cap, that would be swell. So my thinking is that if consumer type receivers/amps have input caps to block dc offset, than I don't need an output cap on my preamp.

Am I smoking crack here, or does this question makes sense? Thanks!
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Old 12th May 2009, 05:01 PM   #2
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If you can reduce the DC voltage on the output to less than 0.5mV or so, then you're ok without the output cap. Otherwise an output cap is highly recommendable.
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Old 12th May 2009, 05:12 PM   #3
AndrewT is offline AndrewT  Scotland
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or fit two outputs:
DC.) direct feed from pre to RCA socket.
AC.) through DC blocking cap (~10uF polypropylene) then to the other RCA socket.
regards Andrew T.
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Old 12th May 2009, 05:46 PM   #4
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AndrewT, is a 10uF PP the best option nowadays? In previous glory days, i would have used a Black Gate NX pro. These days it's harder to find the perfect blocking cap.
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Old 13th May 2009, 06:33 PM   #5
pdul is offline pdul  Denmark
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Capacitors has become a religion nowadays its all about
personal taste. One guy might like the mother, another the daughter and others would want a threesome with both
I guess some experimentation is the answer
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Old 13th May 2009, 06:46 PM   #6
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As long as your OP amp is running off split supply (+/-) voltages
and some means to trim any offset to less than 5-10mv, cap
probably isn't necessary. And then if your amp has cap on input.
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Old 13th May 2009, 07:58 PM   #7
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Originally posted by Lars Clausen
is a 10uF PP the best option nowadays?
To determine the right value for the blocking cap you need to consider the value of Rin. Choose a combination that gives you a low-frequency roll-off low enough not to affect the sonic performance too much. Take into consideration that you will need a DC blocking cap in the feedback leg to ground as well, which forms another high-pass filter. Those two filters will add up and should give a reasonable filter-response, e. g. Butterworth, Bessel, etc. The combination you choose should make sense in your application and harmonize with the connected speakers.

Polypropylene caps are not the best option, but a reasonable option that gives you good value for money.
If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 13th May 2009, 08:28 PM   #8
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A bit depends on what you are running it into.

My amps have DC blocking on the input anyway so dont care what DC voltage is on the input.
Murton-Pike Systems PCBCAD51 pcb design software. http://www.murtonpikesystems.co.uk
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